Techniques have been developed to selectively solubilize cell surface glycoproteins from spontaneous developing murine mammary adenocarcinomas. These cell surface preparations contain antigen(s) which react with monospecific antiserum prepared against the major 52,000 dalton envelope glycoprotein (gp52) of murine mammary tumor virus (MMTV). Fresh tumors of defined pathology from mammary adenocarcinoma patients examined utilizing these techniques demonstrated in a large percentage of samples the presence of an antigen population which cross reacts with MMTV gp52 antisera. No cross reaction was seen in fibroadenomas, a specimen of normal human breast tissue, uterine tumors, and other various benign tumors. Antiserum directed against oncornavirus structural proteins did not react with breast carcinoma cell surface antigens. The reaction was partially competed out (50%) by disrupted MMTV virus. Two dimensional tryptic mapping techniques, although preliminary, suggest that the cross reacting antigen(s) in human carcinoma of the breast share some tryptic peptide homology with MMTV gp52.